1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser having a so-called simplified antiresonant reflecting optical waveguide (hereinafter referred to as S-ARROW) structure, which is constructed so as to confine a basic lateral mode light between a pair of high refractive index layers, each extending with a gap therebetween.
The present invention relates also to a method of fabricating a semiconductor laser having the S-ARROW structure.
2. Description of Related Arts
A semiconductor laser has been predominantly used as a light source for use in optical communication, a light source for use in an optical disc and the like by use of property that a laser light emitted therefrom can be collected up to a diffraction limitation. However, all light radiated from the semiconductor laser can never be collected up to the diffraction limitations thereof. Only light having well matched phases is collected at a light emission end of the semiconductor laser. The semiconductor laser in a condition of being capable of emitting such a light is known as one performing light emission in a basic lateral mode. Under condition that light of various phase is being emitted mixedly, in other words, under condition that high ordered lateral mode light is being mixedly emitted, the light cannot be collected up to a diffraction limitation.
It has been widely known that the foregoing basic lateral mode operation is more stable as an area of a light emission section is made smaller so that it is difficult for the high order lateral mode light to be mixedly present. For this reason, a size of a waveguide path in the semiconductor laser is set to 1 xcexcm or less in a direction of a thickness thereof, and to about 2 to about 4 xcexcm in a direction in parallel with a light emission layer thereof. It has been well known by experience that a semiconductor laser device emitting a light in a basic lateral mode more stably can be manufactured with a high yield as a width of the waveguide path in the direction in parallel with the light emission layer is made narrower.
However, if the area of the light emission section is made small by narrowing the lateral width of the waveguide path, an increase in light density in a light emission end of the semiconductor laser is inevitably brought about. The increase in the light density in the light emission end incurs deterioration of materials constituting the semiconductor laser, resulting in reduction in the life of the device.
In other words, the stabilization of the basic lateral mode (the small section area of the waveguide path) and an increase in light output (the large section area of the waveguide path) are mutually incompatible. Accordingly, to overcome this limitation is an important problem in researching and developing existing semiconductor lasers.
As disclosed in IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 10, No. 8, August 1998, the S-ARROW structure has been proposed as one idea to solve the problem. The S-ARROW structure has a light emission width of about 6 xcexcm, and the width of the light emission section of this structure can be set to be 1.5 to 2 times as large as that of the conventional structure, so that it is anticipated that the maximum light output will be improved.
The reason why the semiconductor laser adopting the S-ARROW structure emits light in the basic lateral mode will be explained below.
A cross section of the principal portion of the semiconductor laser having the S-ARROW structure, that is, a shape in a cross section perpendicular to the waveguide direction, is shown in FIG. 18A. This semiconductor laser comprises an n-GaAs substrate 39; a lower clad layer 38 made of n-InGaP, formed on the n-GaAs substrate 39; a SCH (separate-cofinement-heterostructure) structural layer 37 made of InGaAsP, which includes an InGaAs quantum well activation layer; upper clad layers 36 and 32 made of p-InGaP; an etching stop layer 35 made of n-GaAs; a current blocking layer 34 made of n-AlInP; guide portions 33 of a thickness of e.g. 0.25 xcexcm, made of n-GaAs; and a contact layer 31 made of p-GaAs.
In the above-described structure, since GaAs constituting the above-described guide portion 33 possesses a refractive index higher than that of a periphery of the guide portions 33, an equivalent refractive index in a direction in parallel with the SCH structural layer 37 has a distribution high in the guide portion 33 and low in other portions, as shown in FIG. 18B.
In such a waveguide structure, with respect to the width A of each of the two guide portions 33, a dimension is selected so that while only light in a basic lateral mode is confined between these guide portions 33, light in a high ordered lateral mode is not confined therebetween but escapes outside each of the guide portions 33. According to the literature cited above, the width A of the guide portions 33 is set to 0.85 xcexcm, and the width B of the groove formed by the guide portions 33 is set to 6.5 xcexcm.
Due to the effect of the current blocking layer 34, a current producing a gain of the laser is injected only between the two guide portions 33, and the gain relevant to the laser light is generated only between the guide portions 33.
Accordingly, only the light in the basic lateral mode is confined between the guide portions 33, and hence a sufficient gain can be obtained. On the other hand, since the light in the high order lateral mode is not confined between the guide portions 33, a gain cannot be obtained. As a natural consequence of such a fact, the light in the basic lateral mode is given priority in emission, and the semiconductor laser operates in a stable basic lateral mode until the high light output.
Heretofore, however, the fabrication of the semiconductor laser having the S-ARROW structure has inevitably shown a low yield for the following reason, causing a serious problem in mass production of the semiconductor lasers. To explain this reason, a method of fabricating the semiconductor laser having the S-ARROW structure will first be described with reference to FIGS. 19 to 22.
As shown in FIG. 19, on the n-GaAs substrate 39, there are sequentially grown the lower clad layer 38 made of n-InGaP, the SCH (Separate-confinement-heterostructure) structural layer 37 made of InGaAsP, which includes the InGaAs quantum well activation layer, the upper clad layer 36 made of p-InGaP, the etching stop layer 35 made of n-GaAs, the current blocking layer 34 made of n-AlInP, and the guide portion 33 having a thickness of 0.25 xcexcm, which is made of n-GaAs by use of an organometallic growth method.
The GaAs layer is partially removed by a photolithography step and an etching step while leaving the part of the GaAs layer functioning as the guide portion 33, thus obtaining the sectional structure shown in FIG. 20.
Furthermore, as shown in FIG. 21, a resist pattern 40 is formed at portions other than that corresponding to a groove having the width B (see FIG. 18) by use of a photolithography step.
Thereafter, by use of the resist pattern 40 as a mask, the semiconductor layers including the guide portion 33, the current blocking layer 34 and the etching stop layer 35 are sequentially etched so as to be removed until the upper clad layer 36 made of p-InGaP is exposed. Thus, the sectional structure shown in FIG. 22, in which a pair of resist patterns 40 are formed, is obtained.
Thereafter, the resist pattern 40 is removed, and then the upper clad layer 32 made of p-InGaP and the contact layer 31 made of p-GaAs are formed by a crystal growth method, thus obtaining the semiconductor laser having the structure shown in FIG. 18.
In the conventional method described above, the photolithography steps were performed two times. The pair of resist patterns 40 had a size of about 01 xcexcm and were required to be coincident with each other with a very high precision. If this precision is low, the widths of the guide portions 33 differ from each other, and the guide portions 33 cannot have an original width. Moreover, formation positions of the guide portions 33 deviate from the predetermined positions.
In the semiconductor laser fabricated in such a manner, since light other than that in the basic lateral mode remains between the guide portions 33, the semiconductor laser emits light in a mode other than the basic lateral mode. Furthermore, the light in the basic lateral mode is not sufficiently guided between the guide portions, resulting in a drawback that a threshold value of oscillation current in the basic lateral mode increases. To be concrete, even when the positions and widths of the guide portions 33 deviate from predetermined values by about 0.1 to 0.2 xcexcm, deterioration in electric characteristics of the semiconductor laser is brought about.
The present invention was conceived in view of the foregoing circumstances, and the object is to provide a semiconductor laser having a S-ARROW structure, which has a high precision in shapes of a pair of highly refractive guide portions, thus emitting light in a basic lateral mode stably, and is capable of keeping a threshold value of oscillation current in the basic lateral mode low.
Another object of the present invention is to provide a method of fabricating such a semiconductor laser.
A method of fabricating a semiconductor laser according to the present invention having a structure in which an equivalent refractive index of two portions disposed apart from each other is higher than that of adjacent portions to these portions on a plane perpendicular to a waveguide direction as well as in a direction in parallel with an activation layer, comprises the steps of:
forming the activation layer and a plurality of layers in parallel with the activation layer;
forming a first groove penetrating through at least some of the plurality of layers;
selectively etching a specified layer among the layers through which the first groove penetrates, to a predetermined position toward both sides from the first groove, thus forming a pair of second grooves; and
filling up the second grooves with a material having a refractive index higher than at of the specified layer, thus forming two portions having the high equivalent refractive index.
A first semiconductor laser of the present invention fabricated according to the method, which has a structure in which an equivalent refractive index of two portions disposed apart from each other is higher than that of adjacent portions to these portions in a direction perpendicular to a waveguide direction as well as in parallel with an activation layer, comprises:
the activation layer and a plurality of layers in parallel with the activation layer,
wherein a first groove is formed penetrating through at least some of the plurality of layers;
a pair of second grooves extending from the first groove to predetermined positions toward both sides are formed in a specified layer among the layers through which the first groove penetrates;
the second grooves are filled up with a material having a refractive index higher than that of the specified layer, and thus two portions having the high equivalent refractive index are formed; and
another layer is formed in a state where the two portions contact with the material having the higher refractive index left on a surface portion of the first groove.
A second semiconductor laser of the present invention fabricated according to the method, which has a structure in which an equivalent refractive index of two portions disposed apart from each other is higher than that of adjacent portions to these portions in a direction perpendicular to a waveguide direction as well in parallel with an activation layer, comprises:
the activation layer and a plurality of layers in parallel with the activation layer,
wherein a first groove is formed penetrating through at least some of the plurality of layers;
a pair of second grooves extending from the first groove to predetermined positions toward both sides are formed in a specified layer among the layers through which the first groove penetrates;
the second grooves are filled up with a material having a refractive index higher than that of the specified layer, and thus two portions having the high equivalent refractive index are formed; and
another layer made of a different material from the high refractive material is formed in a state where the two portions contact with a surface portion of the first groove.
A third semiconductor laser of the present invention fabricated according to the method, which has a structure in which an equivalent refractive index of two portions disposed apart from each other is higher than that of adjacent portions to these portions in a direction perpendicular to a waveguide direction as well as in parallel with an activation layer, comprises:
the activation layer and a plurality of layers in parallel with the activation layer,
wherein a first groove is formed penetrating through at least some of the plurality of the layers;
a pair of second grooves extending from the first groove to predetermined positions toward both sides are formed in a specified layer among the layers through which the first groove penetrates;
the second grooves are filled up with a material having a refractive index higher than that of the specified layer; and
layers made of a material having a low refractive index are laminated on the material exposed in the first groove, this refractive index being lower than that of the exposed material, and thus two portions having the high equivalent refractive index are formed outside the material having the low refractive index.
Note that in the third semiconductor laser, the material having the high refractive index and the material having the low refractive index may be two separate materials, each having a constant refractive index, or alternatively one refractive index distribution material in which a refractive index gradually varies in a lamination direction of both materials.
In the method of fabricating each of the semiconductor lasers according to the present invention, the first groove penetrating the layers in parallel with the activation layer is first formed, and then the pair of second grooves are formed by selectively etching the specified layer among the layers through which the first groove penetrates from the first groove to the predetermined positions toward both sides from the first groove. Accordingly, the pair of second grooves are formed at etching rates naturally equal to each other, so depths from the first groove become equal to each other. Thus, widths of the materials having the high refractive index filled up in the pair of the second grooves, that is, widths of guide portions, are formed equal to each other in a self-alignment manner with a high precision.
Accordingly, the semiconductor laser fabricated according to the method of the present invention has the pair of the guide portions made of the high refractive index material, which are formed to shapes with a high precision, and can emit light in a basic lateral mode stably, thus keeping a threshold value of oscillation current in a basic lateral mode low.